The present invention relates generally to gearing capable of providing torque and speed transmission through an angle as required in a helicopter transmission. In particular, the present invention relates to an angular gear drive employing a conical involute pinion, whose tooth surfaces are involute helicoids generated from a base cylinder, in mesh with a mating gear that is theoretically generated by the conical involute pinion.
A typical helicopter transmission includes a large total gear reduction ratio (in the magnitude of 70:1), one or two angular turns from the engine output to the main rotor and, preferably, some power splitting features for increasing the overall capacity within the specified weight and envelope limitations. These same requirements are also found in other applications.
In order to maximize performance of a helicopter, the gear sets comprising the drive train must be carefully selected. The performance of a gear set in transmitting power is judged in terms of, among others, the contact pattern imprinted on the teeth of each member as the members go through the mesh, the backlash of the mating teeth and the transmission errors or conjugacy of the mesh. The relative importance of these factors depends on the arrangement configuration and application requirements of the helicopter or other system driven by the gear set.
The present invention is concerned with gearing drives which transmit speed and torque through an angle. There currently exists a variety of gearing types which may be considered in attempting to accomplish this task. Configuration and efficiency considerations will exclude worm gears, spiroid gearing and helicon gearing from consideration in meeting the high-power intersecting shaft applications associated with helicopter transmissions. This leaves rather limited choices which are discussed below.
The spiral bevel gear set is a common gear arrangement employed in angular gear drives when high speed and high torque applications are desired. Spiral bevel gears have been found not to be suitable when there is a large reduction ratio (&gt;5:1) combined with a large shaft angle resulting in a large pitch angle (&gt;90.degree.) in the gear. These limitations to the use of spiral bevel gearing are due to generation principles and manufacturing methods and are inherent in all spiral bevel gear sets. In addition, spiral bevel gearing is sensitive to establishing the proper contact pattern of the pinion (member with the fewer number of teeth) and the gear (member with the greater number of teeth). This makes it very difficult to adjust the backlash of a spiral bevel gear set without affecting the contact pattern. As a result, creating the proper backlash while retaining the proper contact pattern becomes a tedious, iterative process.
Another gearing type which may be considered are face gears. While this gear type was developed decades ago, it has only recently been considered for high-speed, high-power applications. Technology advancements in the areas of computer modeling and computer numerical control (CNC) machining makes it possible to understand and solve difficult problems in the design and manufacture of face gears such as complex 3D gear geometry, tooth contact analysis, avoidance of undercutting, face gear grinding and coordinate measurement of the tooth profile.
Face gears offer designers an alternative to spiral bevel gears in large shaft-angle, large reduction ratio angular power transmission environments. However, the backlash of conventional face gears can not be adjusted without adversely affecting the contact pattern or the conjugate action between the pinion and the gear. In the real world where manufacturing errors and loaded defections are inevitable, it is extremely difficult, if not impossible, to achieve both the desired contact pattern and the desired backlash during assembly. Maintaining proper tooth contact pattern is vital because the load capacity of a single mesh depends heavily on the proper location and orientation of the contact pattern. Likewise, the proper amount of backlash at each mesh is critical for torque-splitting because, as a closed-loop system, the timing among different power paths has significant impact on the percentage of power share through each power path.
Typical of the systems employing face gears in a helicopter transmission is that shown in U.S. Pat. No. 5,178,028 issued to Bossier, Jr. This patent teaches the use of two concentric, counter-rotating face gears, one being an idler gear and the other an output gear. The gears are in meshing engagement with a driving pinion connected to an engine output shaft. This system clearly does not contemplate the use of a conical involute gear as the pinion in mesh with a mating gear theoretically generated by the conical involute pinion.
In reviewing various types of gearing which could possibly meet the requirement of transmitting power through an angle, one might consider the so-called conical involute gears commonly known as BEVELOID gears, a registered trademark of the Invincible Gear Co., Livonia, Mich. Another name for this type of gearing include tapered involute gears. An advantage of this gearing is its insensitivity to positioning of both the pinion and gear members and its adjustable backlash without violating conjuate action. However, in order to achieve this advantage, conventional conical involute gearing include certain, inherent, drawbacks which make such gearing unsuitable for high power applications. In particular, the cone or taper angle must be relatively small (&lt;10.degree.) or the face width of the gear will be severely limited by pointing at the large end and undercutting at the small end. In addition, the two mating conical gears are always in convex-convex point contact which is a significant limitation on load carrying capacity. During operation of such a gear set, the high relative curvature at the point of contact causes high contact stresses and breaks down desirable lubrication conditions. This problem becomes significantly more serious when the shaft angle is in the range of 70.degree. to 110.degree., which range constitutes a common shaft angle arrangement in helicopter transmissions.
There is a clear and present need for a type of gearing suitable for applications demanding large reduction ratio and large shaft-angle arrangements, while providing appropriate capacity to transmit an appreciable mount of power. Such a gear set should also have the capability of adjusting backlash without affecting the tooth contact pattern and true conjugate action. As will become evident, the present invention provides a unique type of gearing that fulfills all of these needs.